battery discharge

[New Comer]

I have noticed that my AGM batteries will lose a part of its capacity even after a day. I had 13.3v when the battery was fully charged, but next day it went to 13.2 v. I used a battery capacity monitor to monitor the battery and found out that after it was fully charged, it went from 8 notches to 7 notches in a day. That means I lost about 1/8 of the capacity within a day. Is that normal? Or, does this mean the battery is getting old?

Thanks for answering my questions! Because I am new to these sorts of things, I really appreciate your comments.

 

[B and C]

Normal resting voltage is 12.7-8. 13.2 volts sounds like float charge voltage. Resting voltage is measured after the batteries sit for 12 hours with no loads or charging sources.

 

[Seminole Wind]

whatr battery capacity monitor did you use? and what do you mean by "notches" that is not a standard of battery capacity i am familiar with

the nominal voltage of a battery when full charged will be lower than the charging voltage. after the charge source is removed it can take 12-24 hours for the voltage to normalize. it is this normalized voltage that can give you an indication off how charged the battery is. this does not indicate the capacity of the battery other than vaguely in % but not in amp hours available. for example, if we have a 100 amp hour rated battery (20hr rate) and it is old and has degraded to where the capacity is only 75 amp hours. if the battery was still holding full voltage after a charge the could just mean there are 75 amp hours available. still fully charged but some capacity has been lost.

to know the remaining capacity of a used battery compared to the original rating. you have to discharge the battery over time based on what rate the battery was originally rated and see how many amp hours it is still providing at that dis charge rate. this is not a simple test

for the layman if you know your batteries used to run a specific load for a certain time before needing to be recharged but now they need to be recharged sooner when running the same load then that is a good indication of lost capacity in a real world tangible manner

if you want to learn more i highly recommend the compass marine how to web pages. good starting articles on batteries are as follows
https://marinehowto.com/under-load-battery-voltage-vs-soc/
https://marinehowto.com/what-is-a-deep-cycle-battery/

 

[New Comer]

Thank you so much!

 

[Seminole Wind]

no problem. compass marine how to is an immense resource. as you learn and explore more in the world of off grid power you can poke around there and find new aspects to read up on. i go back there all the time

 

[highdesertranger]

yeah something is funny, 13.3 is way to high for resting voltage. now if you just turned the charger off that's another story. highdesertranger

 

[B and C]

I think the capacity monitor is just a voltmeter with the lights substituted for the meter that gives an approximate state of charge like my RV has that has 4 lights.

I use a Trimetric TM-2030-RV to monitor my batteries with. It measures the amps into and out of the battery through a shunt. It is calibrated to the size (Amp hours) (Ah) of my battery bank to provide current voltage, Ah charge/discharge rate and percent of battery remaining. Any meter without a shunt to measure with will not give you a true picture of where your battery is.

 

[New Comer]

I think the capacity monitor is just a voltmeter with the lights substituted for the meter that gives an approximate state of charge like my RV has that has 4 lights.

I use a Trimetric TM-2030-RV to monitor my batteries with.  It measures the amps into and out of the battery through a shunt.  It is calibrated to the size (Amp hours) (Ah) of my battery bank to provide current voltage, Ah charge/discharge rate and percent of battery remaining.  Any meter without a shunt to measure with will not give you a true picture of where your battery is.

Thank you for your answer!